While a packet in the packet transmission is called a "cell" in the ATM (Asynchronous Transfer Mode) communications, this invention relates to a hit-less protection switching method and circuit suitable for use in an ATM exchanger for switching information in cell units or in ATM cross-connect.
In transmission lines having a redundancy construction, protection switching of transmission lines is required for maintenance or the like in a working state. It is desirable in this case to perform a hit-less protection switching so as to avoid data defects in the services being conducted. Conventionally, there is the following hit-less protection switching method as described in "Hit-less Protection Switching Technique for ATM Network" by Tatsuno and Tokura, 1990 Spring National Meeting of Electronics and Telecommunications Academy, B-772 (March, 1990). Specifically explaining, for switching to an emergency transmission line on the transmission side, the transmission side delivers a cell including a switching signal indicative of the last cell on a normal transmission line. The reception side, after receiving this cell, reads cells accumulated in an emergency receiving circuit. The above-mentioned conventional method defines a particular cell indicating a switching signal for performing a hit-less protection switching.